1. Field of the Invention
Exemplary aspects of the present invention generally relate to a drive device that rotatively drives a driven member via a planetary gear reduction device, and an image forming apparatus including the drive device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of a rotatable cylindrical image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor directly onto a sheet or primarily transfers the toner image from the photoconductor onto a rotatable seamless belt (e.g., an intermediate transfer belt) and then secondarily transfers the toner image from the belt onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
The image forming apparatuses often include a drive device that transmits torque from a drive source to the photoconductor or the intermediate transfer belt via a planetary gear reduction device to rotatively drive the photoconductor or the intermediate transfer belt. The planetary gear reduction device has good durability and provides a higher reduction rate with reduced installation space compared to large-sized resin gears and so forth.
Various types of drive devices using the planetary gear reduction device have been proposed. For example, JP-2008-151868-A discloses a drive device 800 that rotatively drives a driven member such as a drive roller 80 for the photoconductor or the intermediate transfer belt as illustrated in FIG. 1. Front and rear portions of the drive roller 80 are held by front and rear holders 81 and 82 via bearings, respectively, and the front and rear holders 81 and 82 are positioned by front and rear lateral plates 83 and 84, respectively. A planetary gear reduction device 85 and a motor 86, which is a drive source for the planetary gear reduction device 85, are disposed within the drive roller 80 and are held by the rear holder 82. An end of the motor 86 opposite an output end thereof is supported by the rear lateral plate 84 via a bearing. Such a configuration allows installation of the drive system within the drive roller 80, thereby reducing installation space of the drive device 800 as a whole and thus providing a more compact image forming apparatus.
In another approach, JP-H10-240069-A discloses a drive device in which a planetary roller reduction mechanism and a motor are provided outside a photoconductor so that output shafts of each of the motor, the photoconductor, and the planetary roller reduction mechanism are arranged coaxially with the other. Accordingly, installation space of the drive device in a direction of diameter of the photoconductor can be reduced, thereby making the image forming apparatus more compact. In addition, in the planetary roller reduction mechanism used for the above-described drive device, only a single side of a support shaft of a planetary roller is supported by a roller support member, thereby reducing installation space of the planetary roller reduction mechanism in an axial direction of the mechanism.
In yet another approach, JP-2001-330087-A discloses a drive device (an actuator) including a planetary gear reduction device. A first sun gear in the first stage of the planetary gear reduction device is extended to an opening formed in an output shaft-side of a housing that holds an outer gear, and sun gears in the second stage and so on are rotatably mounted to the first sun gear. Planetary gears, support shafts of each of the planetary gears, and carriers are assembled in advance as a carrier assembly and then installed in the housing. Each of the support shafts is inserted into needle bearings provided to each of the planetary gears, and both ends of the support shaft, each having a reduced diameter, are sandwiched by first and second lateral plates of the carrier, respectively. The first lateral plate has multiple legs formed together with the first lateral plate as a single integrated component, and the first and second lateral plates are coupled to each other by the legs and the support shafts sandwiched by the first and second lateral plates. Such a configuration allows easy assembly of the planetary gear reduction device and prevents deformation of the carrier assembly, thereby improving durability of each gear.
However, in the drive device 800 disclosed in JP-2008-151868-A, both the motor 86 and the rear holder 82 are positioned by the rear lateral plate 84. The motor 86 is also held by the rear holder 82. In other words, the motor 86 is positioned by the two members, that is, the rear lateral plate 84 and the rear holder 82. Consequently, a variation in the size of each component may cause irregular assembly at a portion where the motor 86 is positioned by the rear lateral plate 84 or deformation of the motor 86 or the rear holder 82. As a result, the planetary gear reduction device 85 and the motor 86 cannot be accurately positioned relative to the driven member such as the photoconductor and the intermediate transfer belt, and the driven member cannot be accurately positioned relative to the rear lateral plate 84, thereby preventing accurate rotation of the driven member.
In the drive device disclosed in JP-H10-240069-A, because only the single side of the support shaft of the planetary roller is supported by the roller support member as described previously, the support shaft of the planetary roller may be tilted. Consequently, the planetary roller may partially contact a sun roller or a stationary roller, thereby degrading durability of the planetary roller and causing rotational fluctuation of the planetary roller. As a result, the drive device cannot accurately drive the driven member such as the photoconductor.
With regard to the drive device disclosed in JP-2001-330087-A, a variation in the size of each component and assembly tolerance may displace a first hole formed in the first lateral plate to hold one end of the support shaft of each of the planetary gears and a second hole formed in the second lateral plate to hold the other end of the support shaft from each other, thereby possibly tilting the planetary gears. The tilt of the planetary gears degrades durability of the planetary gears and causes rotational fluctuation of the planetary gears. As a result, the drive device cannot accurately drive the driven member. In addition, alignment clearance is not provided between each of the carriers and each of the sun gears respectively fitted into input shafts. Consequently, rotational fluctuation caused by tolerances of each of the planetary gears, the sun gears, and the stationary gear is transmitted to the output shaft, thereby preventing accurate driving of the driven member.
Inaccurate driving of the driven member causes speed fluctuation of the driven member such as the photoconductor and the intermediate transfer belt, resulting in jitter or uneven image density in an output image. Continuous speed fluctuation of the photoconductor or the intermediate transfer belt at a certain frequency periodically causes uneven image density throughout the output image, resulting in stripes, or banding. In addition, speed fluctuation of the photoconductor shifts a sub-scanning position of an exposure line and a sub-scanning position upon primary transfer of the toner image from the photoconductor onto the intermediate transfer belt. Further, speed fluctuation of the intermediate transfer belt shifts a sub-scanning position upon secondary transfer of the toner image from the intermediate transfer belt onto the sheet as well as upon primary transfer of the toner image. Thus, banding caused by speed fluctuation of the photoconductor and the intermediate transfer belt considerably degrades image quality.